The proposed program "Molecular Biology of Cellular Interactions" consists of three interrelated areas of investigation: "Cellular Interactions and Growth Control Mechanisms in Cultured Cells" concerns the effects of cell-cell contact on intracellular structures such as polyribosomes, the relationship of contact inhibition with the mitotic cycle, the role of the cell membrane in mediating cellular changes during interaction and the relationship between DNA synthesis and polysome disaggregation. "Cellular Interactions and Differentiation in Cultured Neuronal Cells" involves the study of differentiation of cultured neuroblastoma cells, with special reference to neural specific enzyme activities, viral susceptibility and tumorigenicity. "Mycoplasma Infection of Cultured Cells," is an integral part of our cultured cell studies in that there is a paramount need for maintaining cultures free of extraneous organisms such as mycoplasma. Our work is concentrated on improving our simple, rapid assay for these organisms, comparing its sensitivity to various other detection methods and searching for an effective means of controlling mycoplasma infection in cultured cells.